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I have a PDF document which was scanned from something which I not longer possess.

It is faint, but legible, on screen, but when I print it, it is totally illegible.

How can I increase the sharpness or contrast or make it darker, so that I can print it legibly?

I don't mind converting from PDF to something else - just so long as I get a legible print out.

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Gimp can open a PDF with each page as either a separate layer or as a separate image and has tools that can rescue just about any image but of course if this is a long document you may end up having to spend quite a lot of time on it as you will need to apply the changes to each page manually, (or write a script to do the changes).

However, I would suggest also downloading and installing Image Magick which is a command line image processor that also supports converting from and to the PDF format, and a lot more.

With the 2 free tools above you could convert all the pages to a sequence of images with something like:

convert -density 600 foo.pdf foo-%02d.jpg

pick a typical page and use gimp to discover what changes such as contrast, sharpness, etc., are needed to improve the print quality, (make sure that you only do things that apply to the whole image), then batch process all the images, (from the command line again), to apply those changes to all of the images and reassemble to a new pdf.

Both tools are free, (gratis), and run on a number of platforms including Windows, Mac & Linux.

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I can tell you that this is a long and painful process. You can use ScanTailor.

  • it produces great results
  • freeware
  • cross-platform
  • you need PDF - TIFF conversion and after processing, TIFF to PDF conversion - there are free software solutions for that too
  • you need to check/adjust every page (borders setting)
  • it doesn't like less than 150 DPI images although there is a workaround using XnViewMP, another free software. The preprocessing for ScanTailor is described on my website.

ScanTailor can be used together with XnViewMP. Another tutorial on this subject can be found on my website.

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This can be done easily with XnConvert, and you don't need to break the pdf up into images.

In the output settings, you would tell it to output multi-page pdf to multi-page pdf. In the Actions settings you could set it to Automatic Levels or add the filter for sharpening.

Another program that can do this without breaking up the pdf is NAPS2. You can use this program to run contrast and other filters on the pages of your choice.

However, with both of these programs you will lose any of the pdf's internals bookmark structure. Both programs are free to download and use.

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As time passes by, you can do this with the free GNU Image Manipulation Program GIMP quite comfortably now, if there are not too many pages. With GIMP 2.10, I was able to:

  • Import a PDF file with multiple (9) pages; the pages are "transformed" into layers of the image.
  • Do some brightness and contrast adjustments per page. You have to select each layer in the layer view, located per default on the bottom right. (I have not found a batch processor.)
  • Export to PDF - a dialogue following the file selection asks you to export the layers as pages again.

Et voilà, you're done. You can also save the GIMP project as an .xcf file in case you want to return for additional changes.

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